The effects of the root endophytic fungus Acremonium strictum on plant-herbivore interactions [Elektronische Ressource] / presented by Lara R. Jaber

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• Publié par: georg-august-universitat_gottingen
• Publié le: 01 janvier 2010
• Nombre de lectures: 27
• Langue: English

Extrait

The effects of the root endophytic fungus Acremonium
strictum on plant-herbivore interactions













Dissertation
To obtain the Ph.D. degree
in the Faculty of Agricultural Sciences,
Georg-August-University Göttingen, Germany








Presented by:
Lara R. Jaber
Born in Amman, the Hashemite Kingdom of Jordan






Göttingen, May 2010





























D7
1. Name of referee: Prof. Dr. Stefan Vidal
2. Name of co-referee: Prof. Dr. Teja Tscharntke

thDate of dissertation: May 12 , 2010










In the name of God, Most Gracious, Most Merciful




To Dad
to his spirit that resides in mine and inspires me to aim high all the time
&
Mom
my best friend who made it all possible






Hope I make you proud every single day..Contents 4
Contents ..............................................................................................................4
Summary .............................................................................................................5
Introduction ........................................................................................................7
Objectives...................................................................................14
Chapter I
Interactions between an endophytic fungus, aphids and extrafloral nectaries: do
endophytes induce extrafloral-mediated defences in Vicia faba?
Lara R. Jaber & Stefan Vidal ...............................................................................15
Chapter II
Resource-based trade-off in multiple mutualisms: can nutrient availability shift the
outcomes of multi-species interactions?
Lara R. Jaber & Stefan Vidal ...............................................................................41
Chapter III
Fungal endophyte negative effects on herbivory are enhanced on intact plants and
maintained in a subsequent generation
Lara R. Jaber & Stefan Vidal ...............................................................................80
Discussion ........................................................................................................111
Publications .....................................................................................................123
Acknowledgements .........................................................................................124
Curriculum vitae ............................................................................................125

Summary 5
Summary
The widespread occurence of endophytic fungi in virtually all plant species has prompted an
increasing number of investigations into the ecological significance of these cryptic
microorganisms as mediators of plant-herbivore interactions. In my studies, I investigated the
role of the fungal endophyte Acremonium strictum Gams, restricted to the roots of the
extrafloral (EF) nectary-bearing broad bean plant Vicia faba L., in induction of EF-mediated
defences and reduction of herbivory. In the first experiment, I manipulated the
presence/absence of A. strictum in plant roots and inflicted Aphis fabae damage at a specific
time and location in order to examine whether the endophyte colonization would induce the
EF-mediated indirect defences in response to herbivory. Separately, the endophyte
colonization and the herbivore infestation induced the production of two EF traits (EF nectar
volume and EF nectary number). On the other hand, both EF traits were significantly reduced
in plants simultaneously colonized with the endophyte and infested with the herbivore; which
was predicted (from a cost/benefit perspective) as a trade-off between EF- and endophyte-
mediated defences.
In a subsequent experiment, these interactions were examined under variable levels of nutrient
availabilty. Following herbivory, the level of variation in EF nectar and nectary in the absence
of endophyte infection was only slightly affected by nutrient addition; whereas these EF
rewards responded to nutrient addition in a more complex way in endophyte-infected plants
depending on herbivore damage. Also, increasing nutrient supply increased the extent of root
colonization with A. strictum and alleviated the negative effects of herbivory on plant fitness
in both endophyte-infected and endophyte-free plants. Several measured parameters of the
insect fitness were improved by nutrient addition on endophyte-free plants, but were less
responsive on endophyte-infected plants. Results from this part suggest that plants regulate
multiple mutualisms (i.e. EF- and endophyte-mediated mutualisms) in response to variation in
resource availability so as to attain a favourable cost/benefit ratio. Summary 6
Finally, experiments were conducted to examine whether endophyte effects on herbivory
would depend on the experimental setting used in investigation and whether they would
translate into a subsequent generation of the herbivore. A. strictum negative effects on the
fitness of Helicoverpa armigera first generation were more evident when the larvae foraged
freely on inoculated intact whole plants than when offered leaf discs of inoculated plants, and
these endophyte-mediated negative effects were carried over into the herbivore second
generation. A loss of volatiles or inhibitory effects of compounds that were stronger in situ
might have caused changes in larval feeding and performance on leaf discs as compared to
intact plants, regardless of infection status. Furthermore, the reduction in fitness parameters of
the herbivore across two generations might have been due to the endophyte-triggered
reduction in plant quality.
Results from these studies should have far-reaching conceptual and practical implications for
future endophyte research and should also set the stage for a better understading of the
context under which organisms interact, adapt, and evolve.










Introduction 7
Introduction
Most plant species associate with microbial symbionts (such as mycorrhizal fungi, nitrogen-
fixing bacteria, and fungal endophytes; Smith & Read, 1997; Bacon & White, 2000) which
are increasingly recognized for their potential to influence how their host plants respond to
environmental stresses, including herbivory (Rudgers et al., 2009; and references therein).
The ability or not of fungal endophytes to protect their host plants from herbivory has become
a focus for debate among plant-herbivore ecologists (Clay, 1997; Saikkonen et al., 1998).
Endophytic fungi (sensu Wilson, 1995) have been isolated from all plants studied to date
(Hyde & Soytong, 2008). They are generally categorized as clavicipitaceous (C-endophytes)
and nonclavicipitaceous endophytes (NC-endophytes; see Rodriguez et al., 2009).
Clavicipitaceous endophytes are vertically-transmitted and systemically colonizing the
aboveground parts of grasses, and are best known for their ability to produce alkaloidal
mycotoxins that deter or sicken herbivores (Clay, 1992; Breen, 1994). These grass endophytes
may also benefit their host plants by increasing germination success and plant competitive
abilities (Clay, 1992), in addition to ameliorating the negative effects of drought stress
(Kannadan & Rudgers, 2008). Whereas the clavicipitaceous endophytes in grasses and their
functions are generally thoroughly investigated and well understood, much less work has been
done on the roles of the more ubiquitous nonclavicipitaceous endophytes inhabiting non-grass
host plants (Hyde & Soytong, 2008).
The great abundance and diversity of the unspecialized horizontally-transmitted
nonclavicipitaceous fungal endophytes in woody and herbaceous plants (Petrini, 1986; Petrini
et al., 1992) provide the potential for a wide variety of direct (via mycotoxins; e.g. Findlay et
al., 2003) and indirect (by altering the host plant; e.g. Gaylord et al., 1996; Preszler et al.,
1996; Faeth & Hammon, 1997; Raps & Vidal. 1998) interactions between plants and
herbivores. In addition to their role in increasing resistance to herbivores, nonclavicipitaceous
endophytes have also been implicated in increased disease resistance (e.g. Arnold et al., Introduction 8
2003), increased abiotic stress tolerance (e.g. Rodriguez et al., 2008), and enhancement of
plant growth (e.g. Ernst et al., 2003). However, the generality of mutualism between this
group of fungal endophytes and their non-grass host plants has been questioned because of
inconsistent results from some studies (e.g. Gange, 1996; Faeth & Hammon, 1997; Sieber,
2007). As compared to the clavicipitaceous endophytes in grasses which are generally
considered as plant mutualists (Cheplick & Clay, 1988; Clay, 1992; Clay et al., 1993), there
are three main hypothesis regarding the roles of the nonclavicipitaceous endophytes: (1) that
th